1. Field of the Invention
The present invention relates generally to a versatile integrated multi-sensor apparatus which combines positional data from a variety of sensor types including a GNSS system. The various sensor data is ranked according to its confidence level, and using that data as a means to automatically create a planned path and steer a vehicle along that planned path. Elements of the present invention allow the system to be easily interchangeable among a multitude of vehicles and to communicate with other vehicles to allow for autonomous cooperative vehicle behavior building and task delegation.
2. Description of the Related Art
Global navigation satellite system (GNSS) guidance and control are widely used for vehicle and personal navigation and a variety of other uses involving precision location in geodesic reference systems. GNSS, which includes the Global Positioning System (GPS) and other satellite-based positioning systems, has progressed to sub-centimeter accuracy with known correction techniques, including a number of commercial satellite based augmentation systems (SBASs).
For even more accurate information, higher frequency signals with shorter wavelengths are required. It is known in the art that by using GNSS satellites' carrier phase transmissions, and possibly carrier phase signal components from base reference stations or satellite based augmentation systems (SBAS), including the Wide Area Augmentation System (WAAS) (U.S.), and similar systems such as EGNOS (European Union) and MSAS (Japan), a position may readily be determined to within millimeters. When accomplished with two antennas at a fixed spacing, an angular rotation may be computed using the position differences. In an exemplary embodiment, two antennas placed in the horizontal plane may be employed to compute a heading (rotation about a vertical axis) from a position displacement. Heading information, combined with position, either differentially corrected (DGPS) or carrier phase corrected real-time kinematic (RTK), provides the feedback information desired for a proper control of the vehicle direction.
Another benefit achieved by incorporating a GNSS-based heading sensor is the elimination or reduction of drift and biases resultant from a gyro-only or other inertial sensor approach. Yet another advantage is that heading may be computed while movable equipment is stopped or moving slowly, which is not possible in a single-antenna, GNSS-based approach that requires a velocity vector to derive a heading. Yet another advantage of incorporating a GNSS-based heading sensor is independence from a host vehicle's sensors or additional external sensors. Thus, such a system is readily maintained as equipment-independent and may be moved from one vehicle to another with minimal effort. Yet another exemplary embodiment of the sensor employs global navigation satellite system (GNSS) sensors and measurements to provide accurate, reliable positioning information. GNSS sensors include, but are not limited to, GPS, Global Navigation System (GLONAS), Wide Area Augmentation System (WAAS) and the like, as well as combinations including at least one of the foregoing.
An example of a GNSS is the Global Positioning System (GPS) established by the United States government, which employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 26,500 km. These satellites continually transmit microwave L-band radio signals in two frequency bands, centered at 1575.42 MHz and 1227.6 MHz, denoted as L1 and L2 respectively. These signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites, an ionosphere model and other useful information. GPS receivers process the radio signals, computing ranges to the GPS satellites, and by triangulating these ranges, the GPS receiver determines its position and its internal clock error.
In standalone GPS systems that determine a receiver's antenna position coordinates without reference to a nearby reference receiver, the process of position determination is subject to errors from a number of sources. These include errors in the GPS satellite's clock reference, the location of the orbiting satellite, ionosphere induced propagation delay errors, and troposphere refraction errors. The overall positional signal is weakened with each satellite target lost. These targets may be lost due to obstructions such as trees, hills, or merely because the satellite has orbited out of view.
To overcome these positioning errors of standalone GPS systems, many positioning applications have made use of data from multiple GPS receivers. Typically, in such applications, a reference or base receiver, located at a reference site having known coordinates, receives the GPS satellite signals simultaneously with the receipt of signals by a remote or rover receiver. Depending on the separation distance between the two GPS receivers, many of the errors mentioned above will affect the satellite signals equally for the two receivers. By taking the difference between signals received both at the reference site and the remote location, these errors are effectively eliminated. This facilitates an accurate determination of the remote receiver's coordinates relative to the reference receiver's coordinates. Additional sensors may also be used to support weak GNSS positional data, such as an inertial measurement unit which may include a gyroscope. Such additional sensors are, however, prone to lose calibration and then need to be corrected.
Differential global navigation satellite system (DGNSS) guidance utilizes a localized base receiver of known location in combination with a rover receiver on a moving vehicle for obtaining accurate vehicle positions from GNSS data. Differential positioning, using base and rover receivers, provides more accurate positioning information than standalone systems because the satellite ranging signal transmission errors tend to effect the base and rover receivers equally and therefore can be cancelled out in computing position solutions. In other words, the base-rover position signal “differential” accurately places the rover receiver “relative” to the base receiver. Because the “absolute” geo-reference location of the fixed-position base receiver is precisely known, the absolute position of the rover receiver can be computed using the base receiver known, absolute position and the position of the rover receiver relative thereto.
Differential GPS is well known and exhibits many forms. GPS applications have been improved and enhanced by employing a broader array of satellites such as GNSS and WAAS. For example, see commonly assigned U.S. Pat. No. 6,469,663 to Whitehead et al. titled Method and System for GPS and WAAS Carrier Phase Measurements for Relative Positioning, dated Oct. 22, 2002, the disclosures of which are incorporated by reference herein in their entirety. Additionally, multiple receiver DGPS has been enhanced by utilizing a single receiver to perform differential corrections. For example, see commonly assigned U.S. Pat. No. 6,397,147 to Whitehead titled Relative GPS Positioning Using A Single GPS Receiver With Internally Generated Differential Correction Terms, dated May 28, 2002 the disclosures of which are incorporated by reference herein in their entireties.
It is not uncommon to utilize a GNSS system in combination with an automatic-steering module linked to a vehicle's steering manifold through a steering controller unit. The guidance unit receives positional information from the GNSS unit and compares it with a pre-planned path or map. Because the GNSS positional information allows the guidance unit to know exactly where the vehicle is located along a path, it can use this information to automatically guide and steer the vehicle along this path.
A steering controller is required to accept instructions from the guidance unit and actually perform the steering controls on the vehicle. This device connects to the vehicle steering manifold and/or hydraulic steering valves. Signals from the guidance unit are delivered to the steering controller, which then commands hydraulic valves to open or close depending on the desired results.
Automatic steering systems using GNSS data tend to lose accuracy. If the system calibration is off the steering controller may tend to over-correct, resulting in erratic turns. Additionally, loss of the GNSS signal could affect the automatic steering function.